Today's computer systems are becoming increasingly sophisticated, permitting users to perform an ever greater variety of computing tasks at faster and faster rates. The size of the memory and the speed at which it can be accessed greatly influences upon the overall speed of the computer system.
Generally, the principle underlying the storage of data in a magnetic media (main or mass storage) is the ability to change, and/or reverse, the relative orientation of the magnetization of a storage data bit (i.e. the logic state of a “0” or a “1”). The coercivity of a material is the intensity of the magnetic field needed to reverse the magnetization of a ferromagnetic material along it's easy axis. Generally speaking, the smaller the magnetic particle the higher it's coercivity.
A prior art magnetic memory cell may be spin valve device such as a tunneling magneto-resistance memory cell (TMR), a giant magneto-resistance memory cell (GMR), or a colossal magneto-resistance memory cell (CMR). These types of memory may be commonly referred to as spin valve memory. As shown in prior art FIGS. 1A and 1B a magnetic tunnel junction memory 100 generally includes a data layer 101 (also called a storage layer or bit layer), a reference layer 103, and an intermediate layer 105 between the data layer 101 and the reference layer 103. The data layer 101, the reference layer 103, and the intermediate layer 105 can be made from one or more layers of material.
The data layer 101 is usually a layer of magnetic material that stores a bit of data as an orientation of magnetization M2 that may be altered in response to the application of an external magnetic field or fields. More specifically, the orientation of magnetization M2 of the data layer 101 representing the logic state can be rotated (switched) from a first orientation representing a logic state of “0” to a second orientation, representing a logic state of “1”, and/or vice versa.
The reference layer 103 is usually a layer of magnetic material in which an orientation of magnetization M1 is “pinned”, as in fixed, in a predetermined direction. The direction is predetermined and established using either a hard magnet with a coercivity greater than the free layer or by using an anti-ferromagnetic material (AFM) pinning layer with an appropriate anneal.
The data layer 101 and reference layer 103 may be thought of as stacked bar magnets, where the x axis 107 is longer than the y axis 109, i.e, it has shape anisotropy. The magnetization of each layer has a strong preference to align along the easy axis, generally the long X axis 107. The short Y axis 109 is the hard axis. As with traditional bar magnets, the data layer and reference layer each have magnetic poles, one at either end of the easy axis. The lines of magnetic force that surround the data layer and reference layer respectively are three-dimensional and flow from the North to the South pole.
Typically, the logic state (a “0” or a “1”) of a magnetic memory cell depends on the relative orientations of magnetization in the data layer 101 and the reference layer 103. For example, when an electrical potential bias is applied across the data layer 101 and the reference layer 103 in a SVM 100, electrons migrate between the data layer 101 and the reference layer 103 through the intermediate layer 105. With TMR cells, the material comprising the intermediate layer 105 is typically a thin dielectric layer commonly referred to as a tunnel barrier layer. The phenomena that cause the migration of electrons through the barrier layer may be referred to as quantum mechanical tunneling or, since in FM materials each spin state has a different accessible density of states, spin tunneling.
Continuing with the model of an elemental bar magnet, the magnetization of the data layer 101 is free to rotate, but with a strong preference to align in either direction along the easy axis 107 of the data layer 101. The reference layer 103 likewise is aligned along the easy axis 107 but is pinned in a fixed alignment such that it does not freely rotate in the applied field of interest. The logic state may be determined by measuring the resistance of the memory cell. For example, if the overall orientation of the magnetization in the data layer 101 is parallel to the pinned orientation of magnetization in the reference layer 103 the magnetic memory cell will be in a state of low resistance. If the overall orientation of the magnetization in the data layer 101 is anti-parallel (opposite) to the pinned orientation of magnetization in the reference layer 103 the magnetic memory cell will be in a state of high resistance.
The pinned nature of the reference layer 103 is typically established with the use of an AFM material in direct physical contact with a ferromagnetic (FM) material. AFM materials magnetically order below their Neel temperatures (TN), the temperature at which they become anti-ferromagnetic or anti-ferrimagnetic. The Neel temperature of AFM materials is analogous to the Curie Temperature (TC) of FM materials, the temperature above which a FM loses it's ability to possess an ordered magnetic state in the absence of an external magnetic field. Generally TC of the FM is greater than TN of the AFM.
With respect to a traditional bar magnet there are two equally stable easy spin directions (each rotated 180 degrees) along the easy axis. Alignment in either direction requires the same energy and requires the same external field to align the spin of the atomic particles and thus the magnetic field M1, in either direction as shown by the hysteresis loop 201 for a simple FM in FIG. 2A.
In establishing a reliable pinned field, it is desirable to establish a preferred orientation along one direction of an axis, typically the easy axis although in some situations it may be the hard axis. By growing the FM on an AFM in a magnetic field H or annealing in field H at a temperature above the Neel temperature of the AFM, the hysteresis loop 205 (FM+AFM+H) becomes asymmetric and is shifted, see FIG. 2B. In general, this shift is significantly greater than H, on the order of a couple hundred Oe. This unidirectional shift is called the exchange bias and demonstrates that there is now a preferred easy axis alignment direction.
The annealing step typically takes time, perhaps an hour or more. In the annealing step the reference layer 103 is heated to a temperature greater than TN while a magnetic field is applied. As the temperature is lowered through TN, the spin of the AMF molecules at the interface between the AFM and FM layers will order and couple to the aligned FM spin. Such ordering of the AFM exerts a torque upon the FM material and results in establishing the pinned orientation of the reference layer 103.
As the reference layer 103 is but one part of the memory being produced, the entire memory must be subject to temperatures ranging from about 200 to 300 degrees centigrade while under the influence of an applied magnetic field. Such manufacturing stresses may inadvertently weaken the reference layer 103, leading to an unstable reference field. In addition, the characteristics of the data layer 101 may be unknowingly affected by heat during some manufacturing processes.
The ability to establish the pinned field within the pinned ferromagnetic reference layer 103 is dependent upon the crystalline texture of the ferromagnetic materials and AFM comprising the reference layer 103. Typically, when the pinned reference layer 103 is fabricated on the bottom of the memory structure (a bottom-pinned structure), a layer of suitably lattice matched, generally non-ferromagnetic, material is used to seed the development of a desired crystal structure within a second ferromagnetic seed layer, and the subsequent AFM pinning layer; this texture is propagated in the ferromagnetic reference layer 103. When the AFM layer has been seeded properly, growing the FM layer upon the AFM layer is generally less complicated and consistently results in a desirable crystalline structure
For design and application purposes it is often desirable to have the pinned reference layer 103 above the data layer 101 (a top-pinned structure). This type of structure is difficult to fabricate as the application of the intermediate layer 105 acting as the tunnel junction barrier effectively terminates the propagation of the crystal structure and reapplication of a new non-ferromagnetic seed layer is not possible as the spin dependence is then lost. Growing the AFM layer upon the FM layer (inverse of the order in a bottom-pinned structure) typically involves high energy ion fields and other processes attempting to induce proper crystalline texture. Fabrication of the top reference layer 103 with proper texture in the AFM layer is therefore difficult
In addition, since the AFM layer does not have good texture, the resulting exchange fields are often low, causing the hysteresis loop of the pinned layer to overlap with that of the data layer. This overlap is commonly referred to as the pinned loop spread. As a result of pinned loop spread, the magnetic field applied to switch the state of the data layer must not only be sufficient to overcome the coercivity of the data layer, but also must be sufficient to overcome the influence of the magnetic field overlapping from the pinned reference layer. Furthermore, the required applied fields to switch the bit are asymmetric.
With respect to magnetic memory components, it is well known that as the bit size decreases, it's coercivity increases. A large coercivity is generally undesirable, as it requires a greater magnetic field to be switched, which in turn requires a greater power source and potentially larger switching transistors. Providing large power sources and large switching transistors is generally at odds with the established trends to reduce the size of components. In addition, to mitigate the potential of inadvertently switching a neighboring memory cell, nanometer scaled memory cells are generally more widely spaced relative to their overall size than are non-nanometer sized memory cells. Moreover, as the size of the magnetic memory decreases, the unused space between individual memory cells tends to increase.
Hence, in a typical MRAM array a significant amount of overall space may be used simply to provide a physical buffer between the cells. Absent this buffering space, or otherwise reducing it's ratio, a greater volume of storage in the same physical space could be obtained.
These issues and current design of the magnetic memory cells also carry over into the design and use of magnetic field sensors such as those commonly used in hard drive read cells and read heads. In such implementation, the data layer 101 is termed a sense layer and is oriented by the magnetic field emanating from a storage bit proximate to the read head.
Hence, there is a need for an ultra-high density magnetic memory with a pinned reference layer which overcomes one or more of the drawbacks identified above. The present invention satisfies one or more of these needs.